1. Field of the Invention
The present invention relates to a novel metal powder particularly suitable for a thick film paste and a process for preparing the same, and also to a conductor paste using the metal powder and a multilayer electronic component or a multilayer substrate comprising a conductor formed using the paste.
2. Description of the Prior Art
In the field of electronics, thick film pastes, such as conductor pastes and resistor pastes, have been used for the production of electronic circuits and components, such as resistors, capacitors, and IC packages. The thick film paste is prepared by homogeneously mixing and dispersing a conductive powder, such as a metal, an alloy, or a metal oxide, optionally together with a vitreous binder or other additive(s), in an organic vehicle to prepare a paste. It, after application onto a substrate, is fired at a high temperature to form a conductor film or a resistor film.
Conductive metal powders which are commonly used in the conductor paste or the resistor paste include powders of noble metals, such as silver, gold, platinum, and palladium, and base metals, such as copper, nickel, cobalt, iron, aluminum, molybdenum, and tungsten, or alloys of the above metals.
Among them, in particular, base metal powders are likely to be oxidized during firing and, hence, heated in an oxidizing atmosphere, up to the step of removing the binder at a temperature of usually about 300 to 600.degree. C., to completely burn out the organic vehicle in the paste. Thereafter, they are fired in an inert or reducing atmosphere, such as nitrogen or hydrogen-nitrogen, and, if necessary, the metal, which has been oxidized during the step of removing the binder, is subjected to a reduction treatment. However, oxidation of the base metal to some extent is unavoidable, even by the above method, resulting in deteriorated conductivity and solderability. Further, reduction as the post treatment and strict control of the atmosphere and firing parameters are necessary, leading to a complicated process and increased cost. Therefore, at least the oxidation of the metal powder during the step of removing the binder in an oxidizing atmosphere should be minimized.
In multilayer ceramic electronic components, such as multilayer capacitors, an unfired dielectric layer and an internal conductor paste layer are alternately put on top of the other to form a plurality of layers, and these layer are then co-fired at a high temperature. In this case, ceramics used as a dielectric material, such as barium titanate, titanium oxide, and a composite perovskite containing lead, when fired in a reducing atmosphere, causes an oxygen deficiency, frequently resulting in deteriorated dielectric properties, which poses a reliability problem. Therefore, firing in an oxidizing atmosphere is preferred until the temperature reaches a value at which the ceramic layer is sintered and densified. For this reason, the development of a base metal paste as an internal conductor material, which can be fired in an oxidizing atmosphere, has been desired in the art.
On the other hand, also in the case of a noble metal powder, when a metal (for example, palladium or palladium alloy), which is oxidized at a relatively low temperature and then is reduced by further heating to a higher temperature, is used as an internal conductor of a multilayer ceramic component or as an internal wiring conductor of a multilayer ceramic substrate, volume expansion of the electrode derived from oxidation of the metal and subsequent reduction shrinkage occur during firing. It is known that this volume change is not coincident with the shrinking behavior during the sintering of the ceramic layer and, hence, is likely to cause delamination and cracking. This problem occurs also in the case of the above easily oxidizable base metals, such as copper and nickel. Therefore, in both the noble metal and base metal, the oxidation and reduction of the metal powder during the firing is preferably inhibited as much as possible.
As described in Japanese Patent Publication No. 16041/1985 and Japanese Patent Laid-Open Nos. 131405/1988 and 194137/1990, addition of an element or a metal oxide, which is selectively oxidized during firing, to a conductor paste has been attempted to prevent the oxidation of the metal powder. Depending upon the mixed state of the paste, however, the additive does not effectively act on the surface of the metal powder. Therefore, when the amount of the element or the metal oxide is small, the antioxidation effect is unsatisfactory. On the other hand, addition of a large amount of the element or the metal oxide leads to a deteriorated conductivity and solderability and increased impurities to deteriorate the properties of the electronic components. Further, the metal oxide produced during firing inhibits the sintering of the metal powder, often making it impossible to provide a conductor film having satisfactorily low resistance.
Further, there is a method, as described in Japanese Patent Laid-Open No. 120640/1979, wherein an element, which is preferentially oxidized to form a vitreous material, such as carbon, boron, silicon or aluminum, is alloyed with a conductive metal. However, the alloying element remains in the conductor film after the firing, increasing the resistivity of the resultant conductor. Further, vitrification during the firing requires heat treatment at a temperature considerably above the softening temperature of the glass, making it difficult to control firing conditions and the regulation of the composition and amount. Further, at the present time, there are few processes which can efficiently prepare a fine alloy powder of a submicron size necessary for a reduction in thickness of the conductor layer and a reduction in resistance.
Japanese Patent Laid-Open No. 43504/1992 proposes a method wherein the surface of a metal powder is coated with a metal oxide. This method, however, is unsatisfactory in the antioxidation effect. Further, during the firing, the metal oxide inhibits the sintering of the metal powder. Furthermore, as a matter of fact, it is difficult to effectively coat the surface of the fine metal powder with the metal oxide, and strong aggregation is often created under some treatment conditions.